Hydraulic systems including pumps providing a highly restrictive flow path when not operating, which can include, for instance, but which is not limited to, gear pumps, geroter pumps, vane pumps, and in some instances, piston pumps, are commonly used in a variety of applications, wherein work elements of the systems require pressurization to high pressures, for instance, 3000 psi, for performing work functions such as lifting heavy objects. One such application is the lift and flotation system of an agricultural harvesting machine of the type that requires lifting and supporting a relatively heavy and large header including apparatus for harvesting crops. An exemplary lift system is that of a windrower, although the present discussion is not limited to harvesters of that type, or to harvesters whatsoever.
Some hydraulic systems utilizing gear pumps and other pumps which provide very limited fluid flow capabilities when not operating remain pressurized after being shut down; often with no means to relieve pressure in the system except by the opening of a hydraulic fitting of the system. Also, when shut down, one or more of the work elements may be in a raised position, and further, may be supporting a heavy load. When service or maintenance of such a system is required, service personnel must exercise extreme caution when opening the system to prevent bodily injury and/or equipment damage. Additionally, when the system depressurized rapidly, the service personnel, and other personnel in the vicinity of the system, must beware of possible resultant movement of the work elements of the system and/or objects controlled or supported thereby. In regard to the exemplary lift system for the header of an agricultural harvester, often, the harvester is stopped with the header in an elevated or lifted position, and if system pressure is relieved rapidly, the header may be subject to rapid or abrupt downward movement or dropping. As another potential hazard, escaping high-pressure fluid may be hot, and have sufficient velocity to cause injury. As a result, it is desirable to have a manner of depressurizing a hydraulic system, to avoid the above hazards.
It is known to provide a capability to relieve hydraulic pressure in at least some hydraulic systems of a work machine such as a harvester, with the engine, and most machine controls, operating. It is also known to provide a bleed off valve, for instance, in connection with a hydraulic manifold, to allow manual bleeding off of hydraulic pressure. However, in many applications, the manifold is not in direct fluid communication with all pressurized regions of the hydraulic system, such as at least some versions of header lift and flotation system. There are also instances wherein it is desired to depressurized a hydraulic system of a work machine while the engine is not operating or operable, for instance, during service operations, or as a result of breakdown.
Thus, what is sought is a method of relieving high-pressure conditions in a hydraulic system pressurized by a pump which is highly flow restrictive when not operating, such as a gear pump, particularly, for a work machine such as an agricultural harvester such as a windrower, which avoids the possibility of injury and sudden lowering or dropping of heavy components, without requiring the engine of the machine to be operating.